Astronomers using data from three of NASA’s space telescopes Hubble, Spitzer and Kepler have discovered clear skies and steamy water vapor on a gaseous planet outside our solar system. The planet is about the size of Neptune, making it the smallest planet from which molecules of any kind have beendetected.
John Grunsfeld, assistant administrator of NASA’s Science Mission Directorate in Washington said,
This discovery is a significant milepost on the road to eventually analyzing the atmospheric composition of smaller, rocky planets more like Earth, such achievements are only possible today with the combined capabilities of these unique and powerful observatories.
The planet, HAT-P-11b, is categorized as an exo-Neptune i.e. a Neptune-sized planet that orbits the star HAT-P-11. It is located 120 light-years away in the constellation Cygnus. This planet orbits closer to its star than does our Neptune, making one lap roughly every five days. It is a warm world thought to have a rocky core and gaseous atmosphere. Not much else was known about the composition of the planet, or other exo-Neptunes like it, until now.
In the new study, astronomers set out to look at the atmosphere of HAT-P-11b, not knowing if its weather would call for clouds. They used Hubble’s Wide Field Camera 3, and a technique called transmission spectroscopy, in which a planet is observed as it crosses in front of its parent star. Starlight filters through the rim of the planet’s atmosphere; if molecules like water vapor are present, they absorb some of the starlight, leaving distinct signatures in the light that reaches our telescopes.
Using this strategy, Hubble was able to detect water vapor in HAT-P-11b. But before the team could celebrate clear skies on the exo-Neptune, they had to show that starspots — cooler “freckles” on the face of stars—were not the real sources of water vapor. Cool starspots on the parent star can contain water vapor that might erroneously appear to be from the planet.
The team turned to Kepler and Spitzer. Kepler had been observing one patch of sky for years, and HAT-P-11b happens to lie in the field. Those visible-light data were combined with targeted Spitzer observations taken at infrared wavelengths. By comparing these observations, the astronomers figured out that the starspots were too hot to have any steam. It was at that point the team could celebrate detecting water vapor on a world unlike any in our solar system. This discovery indicates the planet did not have clouds blocking the view, a hopeful sign that more cloudless planets can be located and analyzed in the future.
The results from all three telescopes demonstrate that HAT-P-11b is blanketed in water vapor, hydrogen gas and likely other unidentified molecules. Theorists will be drawing up new models to explain the planet’s makeup and origins.